The Weekly Reflektion 24/2023
The Swiss Cheese model was developed by James Reason at the University of Manchester and is often used in the prevention of major accidents. The model depicts a complex system where hazards are prevented from causing harm by a series of barriers. Each barrier has weaknesses, or holes – hence the similarity with Swiss cheese. Sometimes the number of barriers in place creates an illusion that small holes in the individual barriers may not be so important. This illusion is revealed when the Major Accident occurs.
How small do the holes in the barriers need to be before a Major Accident occurs?
On 8 February 1989, Independent Air Flight 1851, a Boeing 707 on an American charter flight from Bergamo, Italy to Punta Cana, Dominican Republic, struck the mountain Pico Alto while on approach to Santa Maria Airport in the Azores for a stopover. The captain was experienced and had 766 hours on the Boeing 707. The first officer had only 64 hours on the Boeing 707 and was new to Independent Air. The captain was incidentally engaged to one of the flight attendants onboard. The air traffic controller at Santa Marie was a trainee and was being supervised as part of his training.
During the first contact between the airport and the aircraft the flight controller gave the crew a QNH (barometric altimeter setting) of 1018 millibar. This was then used to calibrate the altimeter to ensure correct height measurement on approach. 12 minutes later the controller instructed the air crew that they were cleared to 3000 ft and informed them that the QNH was now 1027 millibar which was incorrect. The supervisor reacted to the error however was distracted by a telephone call, so it was not corrected. The first officer also reacted and asked the captain if the QMH may be in error. A so large change in the QNH was extremely unlikely in such a short time. The captain dismissed the question and the first officer reset the altimeter. The captain was at this time distracted by his fiancé who was in the cabin during the approach to get a nice view of the island. A clear violation of the sterile cabin philosophy.
The first officer responded to the controller with ‘confirm cleared to 2000 feet’. Simultaneously the controlled informed the air crew that they would be landing on runway 19. The radio messages cancelled each other out so the controller did not hear the first officer say 2000 ft. He did not ask the first officer to confirm the 3000 ft clearance height. The aircraft entered clouds on the approach, so visibility was zero. There was turbulence and the captain had difficulty in controlling the aircraft. The Ground Proximity Warning System (GPWS) sounded however the crew took no action. Seven seconds later the aircraft crashed into the mountain at an actual altitude of 1795 feet.
The investigation revealed that during the aircrew training the flight simulations would often give a GPWS warning on landing. The standard procedure for handling this in Independent Air was to ignore or turn off the GPWS. The aircrew were therefore not trained to react immediately to the GPWS alarm on landing.
The aircraft would have clearer the mountain if just one of the following the barriers had been intact:
- the altimeter had been set properly,
- the first officer had heard 3000 ft,
- the controller had asked for confirmation of 3000 ft,
- the captain had reacted to the GPWS,
- communication protocols had been adhered to,
- the captain and the controller’s supervisor had not been distracted.
144 people paid the price of the holes in the barriers with their lives.